Index: test/test_email_renamed.py
===================================================================
--- test/test_email_renamed.py	(revision 54390)
+++ test/test_email_renamed.py	(working copy)
@@ -1537,6 +1537,13 @@
                               ('rg', None), ('\xe5', 'iso-8859-1'),
                               ('sbord', None)])
 
+    def test_i18nheader_unicode(self):
+        h = make_header([('[XXX]', 'us-ascii'), ('Re:', 'us-ascii'),
+                        ('\xbb\xfe', 'euc-jp'), ('\xa9', 'iso-8859-1'),
+                        ('abc', 'us-ascii')])
+        self.assertEqual(unicode(h), u'[XXX] Re: \u6642\u00a9 abc')
+        self.assertEqual(str(h), """\
+[XXX] Re: =?iso-2022-jp?b?GyRCO34bKEI=?= =?iso-8859-1?q?=A9?= abc""")
 
 
 # Test the MIMEMessage class
Index: test/test_email.py
===================================================================
--- test/test_email.py	(revision 54390)
+++ test/test_email.py	(working copy)
@@ -1539,6 +1539,13 @@
                               ('rg', None), ('\xe5', 'iso-8859-1'),
                               ('sbord', None)])
 
+    def test_i18nheader_unicode(self):
+        h = make_header([('[XXX]', 'us-ascii'), ('Re:', 'us-ascii'),
+                        ('\xbb\xfe', 'euc-jp'), ('\xa9', 'iso-8859-1'),
+                        ('abc', 'us-ascii')])
+        self.assertEqual(unicode(h), u'[XXX] Re: \u6642\u00a9 abc')
+        self.assertEqual(str(h), """\
+[XXX] Re: =?iso-2022-jp?b?GyRCO34bKEI=?= =?iso-8859-1?q?=A9?= abc""")
 
 
 # Test the MIMEMessage class
Index: header.py
===================================================================
--- header.py	(revision 54390)
+++ header.py	(working copy)
@@ -199,15 +199,12 @@
         for s, charset in self._chunks:
             # We must preserve spaces between encoded and non-encoded word
             # boundaries, which means for us we need to add a space when we go
-            # from a charset to None/us-ascii, or from None/us-ascii to a
-            # charset.  Only do this for the second and subsequent chunks.
+            # from a charset to None/us-ascii, from None/us-ascii to a
+            # charset, or from None/us-ascii to None/us-ascii.  Only do this
+            # for the second and subsequent chunks.
             nextcs = charset
             if uchunks:
-                if lastcs not in (None, 'us-ascii'):
-                    if nextcs in (None, 'us-ascii'):
-                        uchunks.append(USPACE)
-                        nextcs = None
-                elif nextcs not in (None, 'us-ascii'):
+                if lastcs in (None, 'us-ascii') or nextcs in (None, 'us-ascii'):
                     uchunks.append(USPACE)
             lastcs = nextcs
             uchunks.append(unicode(s, str(charset)))